In the field with which our invention is concerned, the use of capacitive measuring systems for measuring film thickness is known in the art.
One form known in the art is disclosed in U.S. Pat. No. 3,764,899, issued Oct. 9, 1973. This patent measures film thickness by passing the film through a capacitance sensor, wherein variations in film thickness are sensed as dielectric thickness variations between the capacitance members, and are detected as variations in a signal which is applied to the capacitance members. The dimensions and configuration of the "electrodes" or capacitor plates claimed improved characterization of variations in thickness. The patent further claimed a transport for serially examining film material which eliminates errors resulting from tension on the film sample.
Another form of capacitance measuring device is disclosed in U.S. Pat. No. 4,952,882, issued Aug. 28, 1990, which was submitted as an improvement in the construction of the capacitance sensor disclosed in the foregoing United States patent. This capacitance sensor assembly also measures changes in thickness of a dielectric film, such as plastic film, where the sensor components are constructed from materials having low coefficients of linear temperature expansion, resulting in a claimed measurement device which provides low measurement errors resulting from temperature effects on material expansion and dielectric changes. The design concept was to minimize, not eliminate, the errors caused by temperature effects on the sensor assembly.
Another form of capacitance measuring device is disclosed in U.S. Pat. No. 3,471,780, issued Oct. 7, 1969. This capacitance sensor assembly also measures changes in thickness of a dielectric film, such as plastic film, where the sensor design was a single capacitive cell with three capacitor plates, which utilized materials of construction and component dimensions which were claimed to minimize the effects of temperature variations and their related changes in dimensional and dielectric properties of the cell. Moisture effects were claimed to be negated by manually adjusting a micrometer which would move the secondary capacitive plate (center plate of three plates) in the cell to counterbalance the effects of moisture in the atmosphere or film sample. However, there was no allowance to monitor the effects of environmental changes (alone) nor an automatic method to eliminate them from the thickness readings.